This invention relates to a vehicular navigation system for calculating the traveling distance and heading of a vehicle with a high precision, sensing the present position of the vehicle and providing guidance from the sensed present position to a desired destination.
In a vehicular navigation system, a distance sensor is used to sense a number of pulses outputted in proportion to the rotation of a tire, by way of example, and distance traveled is calculated by multiplying the number of pulses by a distance per unit pulse. The angle of rotation of the vehicle is calculated using a heading sensor, the present position of the vehicle is determined from the distance traveled and the angle of rotation, and the present position of the vehicle is collated with map data and displayed on a display unit. In this case, use may also be made of a global positioning system (GPS) to determine the absolute position of the vehicle for the purpose of correcting the present position.
Tire diameter changes with aging of the tire and with a change in air pressure inside the tire. Consequently, in the above-described method of calculating traveling distance, it is required that the distance per unit pulse be corrected when the tire is replaced in order to assure accuracy in distance measurement. To this end, calculation includes multiplication by a distance correction coefficient. However, the operation for effecting the correction by the distance correction coefficient is very difficult for the user of the navigation system to perform.
In order to solve this problem, the specification of Japanese Patent Application Laid-Open No. 5-209756 proposes obtaining the linear distance between two locations measured by a GPS and the linear distance between estimated positions at these two locations, and revising the distance correction coefficient in such a manner that these two linear distances will coincide.
In the method of performing the distance correction using the distance between two points, namely the present position and the immediately preceding position, by obtaining positional coordinating from a GPS as in the above-mentioned Japanese Patent Application Laid Open No. 5-209756, the position measured by the GPS fluctuates owing to the disposition of the satellite and the state of radio wave reception. As a result, in order to assure a certain degree of accuracy in the calculation of the distance correction coefficient, it is required that the number of samplings be increased.
However, in a case where the distance between two points is used, a fixed distance must be traveled to assure a certain degree of accuracy. This means a longer sampling interval and a smaller number of samplings. Furthermore, in a case where a filter is provided to filter out samplings having a poor reception precision for the purpose of improving accuracy, or in a case where radio waves cannot be received because the vehicle is between mountains or in the shadow of buildings, the number of samplings is reduced. This makes it necessary to take a large number of samplings over short intervals. However, shortening the sampling interval, i.e., sampling the distance between the two points, means greatly diminishing precision.
With regard to detection of heading, the specification of Japanese Patent Application Laid-Open No. 6-66577 discloses a vehicular navigation system in which heading is sensed by a geomagnetic sensor for sensing geomagnetism, left-right wheel sensors for sensing a difference between the number of revolutions of the left and right front wheels of the vehicle in order to revise an error in the geomagnetic sensor which can occur when there is a disturbance in the magnetic field, and a steering sensor for applying compensation in a case where the left-right wheel sensors become inoperative at low speeds of less than 3.5 km/h. This is supplemented by estimating the heading of the vehicle using a GPS in which position is sensed by receiving radio waves from an artificial satellite. When a road nearest to the present position of the vehicle has been selected by map matching, the bearing of this road, which has been stored as map information, and the estimated heading are compared and a vehicle heading which coincides with the road bearing is decided. In a navigation system of this kind, radio waves from the GPS satellite are received and used to calculating heading. However, if the vehicle is traveling at low speed or is executing a left or right turn, a sufficient accuracy cannot be obtained with regard to determination of heading by the Doppler effect. In such case the GPS heading is not employed.
In detection of heading in the conventional navigation system, a GPS heading is employed only when the vehicle speed is greater than a predetermined value, in view of the fact that heading accuracy declines if the vehicle is traveling at low speed or is executing a left or right turn. As a consequence, correction of heading by employing the GPS heading becomes less frequent and heading accuracy declines as a result. Further, even if the vehicle speed is greater than the predetermined value, there are occasions where the accuracy of the GPS heading declines, as when the disposition of the GPS satellite or the radio wave reception is poor. If heading is corrected using the GPS heading in such case, the heading may instead be corrected to the wrong heading and heading accuracy will be diminished as a result.